1. Field of the Invention
The invention relates to flowmeters and more particularly to a device for performing flow rectification mainly in one plane such as for use with a vortex flowmeter.
2. Description of the Prior Art
Fluid flowmeters utilizing an annular flow element, a paddle wheel, insertion and vortex flowmeters are examples of flowmeters which require that the approaching flow stream be conditioned or rectified in at least one plane. Because a vortex flowmeter amplifies the effect of flow irregularities present with most of these meters, it is a good example to use in explaining flow rectification in one plane.
The process of vortex shedding is based on stimulating existing hydrodynamic instabilities when a bluff body is placed in the flow stream over a certain Reynolds number range. The alternation of the shedding process to form the Karman Vortex street depends on interaction of the two shear layers on the two sides of the wake behind the body. This is why a vortex flowmeter is more sensitive to flow irregularities than some other types of flowmeters.
The flow irregularities of the approaching stream are produced by pipework upstream of the flowmeter. The installation of the meter downstream of pipe fittings such as valves, elbows and reducers results in flow irregularities. Meter misalignment, which leaves steps and gaps, and the protrusion of a gasket into the flow stream at the meter inlet, also lead to flow irregularities. The irregularities, due to pipe fitting, are usually handled by using a different pipework, allowing a certain minimum straight pipe length upstream of the meter, or using a flow straightener. Currently, none of these methods correct the problems caused by meter misalignment or eccentricity or flange problems at the meter inlet. Modification of the pipework and/or the provision of a long straight upstream pipe is sometimes impossible.
The use of flow straighteners upstream of the meter has been suggested to correct particular types of flow irregularities. Basically, there are four types of flow straighteners: the rectangular "egg crate" type known as the AMCA straightener, radial plates meeting at the pipe center known as the Etoile straightener, a bundle of tubes each having their axes parallel to the pipe, and a perforated plate type straightener.
As mentioned above, such flow straighteners do not correct irregularities that occur at the meter inlet due to gaskets, steps and gaps, eccentricity and misalignment. Straighteners must be placed at distances upstream the sensor element that depend on their type and the resulting interaction between the straightener and the sensor element in order to give accurate results. The wake of a flow straightener can adversely affect the sensor element. Some straighteners with lower pressure drop and shorter rectification distance can be placed closer to the meter. For example, U.S. Pat. No. 4,397,192 outlines the use of a mesh made of bars at the inlet section of a vortex meter for the purpose of improving its linearity, especially at Reynolds numbers less than 20,000.
It has been argued that certain level of upstream turbulence are favorable, especially when they are reproducible. This effect has been used in the case of a vortex flowmeter by Pankanin and Tyszkiewic to develop a shedding body based on what is known as the sunken stream effect (see Proceedings of FLOMEKO 1983, IMEKO).
It has been observed that the vortex shedding process requires rectification of the flow profile in the plane perpendicular to the axis of the shedding body such that the flow becomes symmetric with respect to the lines of separation where the vortices detach. The shedding process tolerates to a great extent the nonuniformity in the velocity profile along the separation edge. In part, this is because the vortex theorems of Helmoholtz state that a vortex filament must be either a closed tube or end on the boundaries of the fluid. The vortex tube tries to keep its coherence along the axis of rotation and resists the tearing effects when there is a tendency to form more than one vortex cell along the shedder bar. Most vortex flowmeters have shedders with a length to face width ratio (1/d) in the neighborhood of three. The published data (see Mair and Stansby, SIAM J. Appl. Math., 28(2), 519-540,1975) on a very slender shedding body (1/d=10) show that, even when the skewness in the flow profile along the shedding body is severe enough to produce a number of shedding cells, the cell sizes fit segments of the shedding body having 1/d=3. In other words, with 1/d=3, a very strong correlation along the axis of rotation of the shed vortex prevents it from breaking into several cells.